1. Field of the Invention
The invention relates to the fabrication of gas turbine engine wheels which are a composite of carbon cloth, chopped carbon fibers, carbon bearing resin and elongated carbon fibers extending continuously through the hub and blades.
2. Description of the Prior Art
It is desirable for the efficiency of a gas turbine engine to have the combustion gas temperature at the maximum level that the turbine and nozzle structures can withstand. Currently, metal turbine wheels permit a maximum turbine inlet temperature of approximately 2000.degree. F. An increase in this temperature requires use of advanced materials such as ceramics or carbon. The maximum useful service temperature of carbon, when coated, is approximately 6000.degree. F. However, in block or disc form, it has an absolute strength which is well below requirements. Filament or fiber forms of carbon, on the other hand, have a high tensile strength, perhaps double or triple that of metals. A solution to the low strength problem is thus to weave carbon fibers into cloth which is then laid up in disc shape and densified by chemical vapor deposition of carbon. This type of structure is known as a carbon/carbon composite.
A major problem associated with the manufacture of such composite turbine wheels is that of orienting the carbon strengthening fibers so that they will carry most of the centrifugal and thermal stresses generated in the bladed turbine wheel. In the blade and neck area of the wheel it is desirable to have a radial orientation of the fibers because of the nature of the stresses. However in the bore or hub area a circumferential orientation of the fibers is preferable. There is presently no known procedure for accomplishing this overall construction.
The second problem with the carbon/carbon composite turbine wheels is that the blades have a complex airfoil shape which must be machined into the periphery of the disc without severing any of the carbon strengthening fibers. If cut, the fibers lose their strength and the resulting wheel may be unsuitable for use. No presently known procedure exists for allowing the blades to be machined without severing fibers.
It is known to manufacture composite structures such as fiberglass reinforced plastics utilizing the tensile strength of filaments in the blades of compressor rotors. It is also known to use a combination of amorphous carbon plus carbon filaments in such products as aircraft brake discs, turbine impellers and re-entry heat shields.